Polysiloxane emulsions may be categorized by the size of the polysiloxane particles and the appearance of the emulsion. Typically three categories of silicone emulsions are recognized in the art--standard emulsions, fine emulsions and microemulsions. The term emulsion used herein encompasses the three individual types of silicone emulsions.
Silicone standard emulsions are characterized by a large particle size (typically greater than 300 nanometers) and appear to the human eye to be opaque (impenetrable to light). Silicone standard emulsions are most commonly identified as those having an intense white appearance. Silicone fine emulsions are characterized by a smaller particle size, from 300 to 140 nanometers, and are identified as those compositions which visually may be slightly opaque to very slightly translucent (transmitting light but with distortion). Silicone microemulsions are characterized as those having a particle size of less than 140 nanometers and visually appear translucent to transparent (transmitting light without distortion).
Out of the three types of silicone emulsions, fine emulsions and microemulsions are the most desired due to their smaller particle size and higher stability. Microemulsions are further desired due to their translucent to transparent appearance. Because of these factors, microemulsions are more preferred and can be used in more applications than standard or fine emulsions.
Methods for making emulsions of polysiloxanes in water are well known in the art. The methods are generally classified in two categories: mechanical means and emulsion polymerization. Mechanical means usually entail taking the polysiloxane and using mechanical means such as homogenizers or vigorous agitation to emulsify the siloxanes in water. Typically a surfactant is added to the polysiloxane or water to aid the emulsification process.
A mechanical method for making fine emulsions and microemulsions is taught in U.S. Pat. No. 4,620,878 to Gee. The method involves forming a "translucent concentrate" of surfactant, polysiloxane, and water in select proportions. The "concentrate" is then rapidly dispersed in additional water to form the fine or micro emulsion.
A mechanical method for producing standard emulsions is taught in U.S. Pat. No. 2,755,194 to Volkmann et al. The method taught by Volkmann et al. involves mixing an emulsifying agent, polysiloxane and small amount of water. The mixture is then processed by milling or mixing. This mixture is then added to additional water with appropriate mixing techniques employed to produce a stable emulsion. Alternatively all ingredients may be combined in a single step and mixed.
U.S. Pat. Nos. 3,975,294 and 4,052,331 to Dumoulin teaches emulsifying agents which may be used to create transparent microemulsions using mechanical means. These microemulsions can contain up to 15% by weight of the polysiloxane. Because the microemulsions are prepared using mechanical means only low molecular weight (less than 20,000) polysiloxanes can be emulsified.
Rosano teaches two methods for preparing microemulsions in U.S. Pat. Nos. 4,146,499 and 4,472,291. U.S. Patent No. '499 teaches a method where a surfactant and oil (polydi-organosiloxane) are combined and added to a water and surfactant solution. U.S. Patent No. '291 teaches a method for forming high viscosity microemulsions. The high viscosity microemulsions are prepared by first forming a "low viscosity" microemulsion using mechanical means and then adding an additional surfactant to that microemulsion which increases the viscosity.
Emulsion polymerization typically entails combining a reactive silicone oligomer, surfactant, polymerization catalyst and water. The mixture is stirred and the silicone oligomers are allowed to polymerize until a standard emulsion, fine emulsion or microemulsion is formed. Typically alkoxysilanes, which result in the formation of microemulsions, or cyclicsiloxanes, which result in the formation of fine and standard emulsions are used as the reactive monomers and oligomers. Combinations of the silicone reactants can also be used to form copolymers in the resulting emulsion.
Typical problems encountered with emulsion polymerization of cyclicsiloxanes include the presence of an unemulsified silicone oil layer or very large (visible to the human eye) silicone oil droplets in the final emulsion produced. Using methods known in the art, complete elimination of the silicone oil layer is not achieved unless the cyclicsiloxane is pre-emulsified using mechanical means prior to polymerization. Mechanical pre-emulsification of the cyclicsiloxanes in water prior to emulsion polymerization is a common, well known practice to those skilled in the art. Also, when using cyclicsiloxanes as the starting oligomer in methods known in the art, it is generally not possible to achieve a conversion of cyclicsiloxane into polysiloxane polymer greater than amounts determined by the ring/chain equilibrium of the system. In other words, there will always be a certain level of cyclicsiloxane present in the emulsion and this level is typically determined by the ring/chain equilibrium for the system.
There are numerous pieces of art which teach methods for using emulsion polymerization. For example U.S. Pat. No. 2,891,920 to Hyde et al. teaches an emulsion polymerization method where the polysiloxane, emulsifying agent (cationic, anionic OR nonionic surfactant), catalyst and water are all blended together (in various orders) to form an emulsion and then allowed to react at room temperature or greater. It appears that it is possible to produce only standard and possibly fine emulsions by this method.
U.S. Pat. No. 3,294,725 to Findlay et al. teaches an emulsion polymerization method wherein a mechanically produced pre-emulsion is made of the siloxane in the presence of the catalyst. Heat is then applied to this emulsion and the siloxanes reacts to form the polysiloxane emulsion. Various sulfonic acids and their salts are taught as the catalysts for the polymerization. The catalysts also act as the emulsifying agent thereby eliminated the need for additional materials. The examples provided in the Findlay patent show that when cyclicsiloxanes are employed as the starting material, less than 90% of the starting cyclicsiloxanes are consumed after several days of reacting. The resulting emulsions contain a broad distribution of particle sizes of polysiloxanes and may contain a layer of unemulsified silicone oil if the polysiloxane is not pre-emulsified. It appears that only standard and fine emulsions can be produced by this method.
A method for preparing colloidal suspensions of only silsesquioxanes is taught in U.S. Pat. No. 3,433,780 to Cekada et al. These colloidal suspension have an extremely small particle size (10 nanometers to 100 nanometers) and in most cases contain less than 25% by weight of the silsesquioxane. The method comprises combining the water and catalyst, heating the water solution (optional) and rapidly or slowly adding a trialkoxysilane to the water solution. When rapid addition is employed the suspension can contain up to 10% by weight of the silsesquioxane. Silsesquioxane are materials which contain 3 Si--O bonds per molecule.
Several papers have been published in China on studies of silicone emulsion polymerization using cationic surfactants and in some experiments, additionally using nonionic surfactants. A paper published by Northwestern University of Light Industry, China, Xibei Qingongye Xueyuan Xuebao, No. 4, pp. 5-10, December 1987, discusses the improved stability of emulsion produced using a nonionic surfactant and an anionic surfactant. However, the method employed in the paper does not result in an "oil free" emulsion with the least amount of unemulsified oil cited as being 0.9%. Nor does the paper teach a method which allows for controlling the particle size.
Additionally, two papers, Institute of Chemistry, Academia Sinica, Beijing, China, Polymer Communications, No. Aug. 4, 1982, pp. 266-270 and pp. 310-313, report on the mechanism of cationic emulsion polymerization and the effect of temperature on cationic emulsion polymerization, respectively. The emulsions taught in these papers do not contain nonionic surfactants and again do not teach a method which allows for specific control of the particle size.
The methods known in the art using emulsion polymerization are limited as the resulting emulsions do not have a broad range of possible particle sizes. Further, the methods have narrow or no range in which they will produce oil-free (those containing no unemulsified silicone oil) emulsions unless pre-emulsification is employed.
It is an object of this invention to provide a method for making polysiloxane emulsions using emulsion polymerization.
It is further an object of this invention to provide a method for controlling the emulsion polymerization to produce a desired type of emulsion having a desired particle size and which is free of unemulsified silicone oil.
It is further an object of this invention to provide a method for producing stable, oil-free microemulsions without having to mechanically pre-emulsify the polysiloxane.
It is further an object of this invention to provide a single method which allows for the production of emulsions, fine emulsions and microemulsions.